Vibrational motion of a single molecule
measured in real time
20 August 2014
For the first time, chemists have succeeded in
measuring vibrational motion of a single molecule
with a femtosecond time resolution. The study
reveals how vibration of a single molecule differs
from the behaviour of larger molecular groups.
Nanoparticle amplification of molecular signals is
employed in various spectroscopic techniques,
such as surface-enhanced Raman scattering
(SERS), which is used for routine detection of
single molecules.
The study was performed at the University of
California, Irvine, where post-doctoral researcher
Eero Hulkko from the University of Jyväskylä works
as a visiting fellow under professor Vartkess A.
Apkarian, whose team participated in the study.
The second team was lead by Professor Eric O.
Potma. The results of the study made the cover of
the August issue of the prestigious Nature
Photonics magazine.
Vibrations in a single molecule are strictly governed
by quantum mechanics. In order to be detectable
with the method, a molecule has to be in two (or
more) quantum states at once. In quantum
mechanics, this is known as coherent superposition
of vibrational states – a wave packet. In molecular
groups, the states within the wave packet usually
lose their phase correlation within a short period of
time. This is called dephasing.
The scientists employed ultra-short femtosecond
(10-15 s) laser pulses in the visible light spectrum
to measure the motion of individual molecules.
Lack of dephasing observed for the first time
In conjunction with the study, the scientists
developed a new method for identifying single
molecules by optical means.
The tests proved that the vibrations of a single
molecule did not lose their phase correlation. Using
simulations, the scientists were able to explain the
observations and show that pure dephasing is a
characteristic of molecular groups. The lack of
dephasing in single molecules is a fundamental and
hitherto unknown finding.
With the aid of laser pulses, the scientists were
able to create a wave packet of the single molecule
vibrations and monitor it during 10 picoseconds (1
Seeing a single organic bipyridylethylene (BPE)
molecule vibrate as a function of time was possible picosecond = 10 to the power of -12 seconds). The
time dependent motion of the wave packet
through the scattering of the light pulses. The
corresponds to the actual vibrations of the
method is known as time-resolved coherent antimolecule, i.e. its molecular "breathing".
Stokes Raman scattering (tr-CARS).
Aided by gold nanoantennas
The observations of single BPE molecule
vibrations was possible using so called "plasmonic
nanoantennas", which consist of two gold
nanoparticles of approximately 90 nanometres. A
nanometre is one billionth of a meter. The
nanoantennas amplify the radiation from a single
molecule to a detectable level.
"Observing the breathing of a single molecule takes
us one step closer to seeing real chemistry at work
at the level of a single molecule", Professor
Apkarian predicts.
Demonstrating coherent ultrafast spectroscopy at
"Detecting one molecule through scattering of light the single-molecule level – in other words, being
able to generate, manipulate and measure the
is extremely difficult", says Doctor Hulkko, who
participated in the study, "which is why we needed quantum states of a single molecule – opens up
new possibilities for quantum computing using
to amplify the signal".
1/2
molecules and quantum information transfer.
In practice, single molecules means that we are
observing individual photons. Consequently, the
results open new research avenues and
applications within single-molecule photonics, such
as generating single photons in a desired state.
More information: Steven Yampolsky, Dmitry
A.Fishman, Shirshendu Dey, Eero Hulkko, Mayukh
Banik, Eric O. Potma & Vartkess A. Apkarian,
"Seeing a single molecule vibrate through timeresolved coherent anti-Stokes Raman scattering",
Nature Photonics, 8, 650-656 (2014), DOI:
10.1038/nphoton.2014.143
Perspective: Lukasz Piatkowski, James T. Hugall &
Niek F. van Hulst, "Raman spectroscopy: Watching
a molecule breathe" 8, 589-591 (2014), DOI:
10.1038/nphoton.2014.174
Provided by Academy of Finland
APA citation: Vibrational motion of a single molecule measured in real time (2014, August 20) retrieved
16 June 2017 from https://phys.org/news/2014-08-vibrational-motion-molecule-real.html
This document is subject to copyright. Apart from any fair dealing for the purpose of private study or research, no
part may be reproduced without the written permission. The content is provided for information purposes only.
2/2
Powered by TCPDF (www.tcpdf.org)